Does it even need a template or could you just take the base character's stats and change their alignment to evil (I figure the law/chaos part of their alignment remains the same.)
Yeah just make them evil, no changes required.
If the creature you're changing is a Paladin you might want to take a look at the Antipaladin, but mostly this is going to be just a flavour reskin.
The Trompe l'Oeil comes to mind.
Do it like Dark Link from Legend of Zelda and give him Shadow Template or Shadow-traced Creature
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The first thing that came to mind is the Alter Ego Template.
The template is used to create a copy of the initial creature, but making it a construct with copied classlevels, higher dex and a distance dependacy with the original.